If a broader access bandwidth to a memory is desired, there are two methods to achieve this. The first is to increase the clock frequency for accessing the memory. The second is to expand the bus width in order to increase the amount of data that can be accessed at a time. A clock frequency cannot be increased to exceed the maximum frequency supported by a memory. Therefore, an access bandwidth has generally been broadened by first increasing the memory frequency to the limit, and thereafter by expanding the bus width to increase the amount of data accessed at a time.
One example of related art of such a memory access method is the one disclosed in Patent Literature 1 (Japanese Patent Laying-Open No. 1998-210251). The art of Patent Literature 1 divides an entire image into blocks, and forms each block with data of burst-accessible memory addresses. Within one block, any data arrays, whether lateral, longitudinal or otherwise, can be burst accessed, thereby enabling fast access to an image memory regardless of the direction of accesses.
Patent Literature 1: Japanese Patent Laying-Open No. 1998-210251
However, memory access methods according to related arts, including the one disclosed in Patent Literature 1, have a problem in that access efficiency is decreased. This is because when the bus width is increased, the amount of data assigned to one address also increases, causing data other than the desired one to be unnecessarily accessed at the same time.
For example, when two-dimensional image data are accessed, the data can be accessed in two directions: lateral (horizontal) and longitudinal (vertical). If an address stores laterally consecutive data, but if the user wants to access longitudinally consecutive data, such access could be extremely inefficient because many data pieces are unnecessarily accessed. Similarly, when accessing an address storing data that are longitudinally consecutive, on the contrary, if the user wants to access laterally consecutive data, access efficiency tends to be very low because a lot of unnecessary data are accessed.
(Object)
The object of the present invention is to increase memory access efficiency when accessing data stored in a memory.